Introduction

This article presents the formation of monodisperse double emulsion droplets in a single step using a microfluidic device featuring a surface functionalized engineered surface. The system used for the double emulsion production is Dolomite’s Double Emulsion System.

The resulting double emulsions consist of water with sodium dodecyl sulphate (SDS) as inner phase, mineral oil with polyglycerol polyricinoleate (PGPR) as middle phase, and water with polysorbate 80 (Tween-80) as outer phase.

In this case, emulsifiers analyzed are widely used in cosmetic applications as well as approved for pharmaceutical applications in their respective purity grades. Double emulsions made up of an aqueous core are shown in Figure 1.

Figure 1. Double emulsions made up of an aqueous core (w), an organic shell (o), and an aqueous continuous fluid (w).

The P-Pump helps tuning the droplet size by controlling the fluid. Dolomite’s Double Emulsion Chip with etch depth of 100µm and its Mitos P-Pump are used to produce double emulsions.

Pulseless liquid flow is delivered by the P-Pumps with a precise pressure driven pumping mechanism. P-Pumps drive the fluid by displacement using compressed air or inert gas, thus delivering ultra-smooth flow.

This precise flow is crucial for achieving monodispersity. Using helium addresses the occasional problems arisen with dissolved gases, thanks to its very low solubility in liquids. Flow rate monitoring and detection of backflow events can be carried out with Mitos Flow Rate Sensors (Part No. 3200098 and 3200099) and Mitos Sensor Display (Part No. 3200095).

Additional flow resistance can be introduced into the system by the F10 Flow Resistor (Part No. 3200269) and F30 Flow Resistor (Part No. 3200270). The use of flow resistances leads to a broader operating range and helps achieving better control on modulating flow ratios.

The TConnector ETFE (Part No. 3000397) and the 2-way In-line Valve (Part No. 3200087) facilitate sequential introduction of the fluid onto the chip, thus helping to explore optimal pressures and flow rates during start-up. The chip junction is viewed using a High Speed Camera and Microscope System (Part No. 3200050).

Frame grabbing at higher frequencies of up to 1.5kHz can be achieved by decreasing the pixel size of the image. Dolomite’s Droplet Monitor software is used to analyze the videos recorded to calculate droplet production rates and outer droplet size.

The schematic of the Double Emulsion Chip with two fluidic junctions is depicted in Figure 2. The upper junction is used for double emulsion production, while the lower junction is used for single emulsion production. This article highlights the application of the upper junction.

Figure 2. Schematic of the Double Emulsion Chip

Experimental Setup

The schematic of the experimental setup is shown in Figure 3. When tubing of different inner diameters are connected, it is recommended to use smaller ID tubing downstream and larger ID tubing upstream leading to the chip.

The fluids to be pumped into the chip include 1% SDS in water (inner fluid), 2 % PGPR in mineral oil (middle fluid), and 2% Tween 80 in water (outer continuous fluid).

Figure 3. Schematic of the experimental setup

The solutions are freshly prepared in advance, and degassed to avoid the introduction of bubbles into the system. They are further filtered by passing through a 0.2µm pore size filter.

The system must be carefully assembled to ensure minimal introduction of dust into the system. At least one Ferrule with Integrated Filter (Part No. 3200245) is employed wherever possible as an in-line ferrule in each of the fluidic tubing directing to the chip.

Experimental Results

Flow Characteristics of the Double Emulsion System

In pressure driven pumping, the flow rate Q varies proportionally with the set pressure P in the pump and inversely with the fluid resistance Rf. The fluid resistance is influenced by fluid properties such as viscosity, and structural characteristics such as length of tubing and cross sectional area of the flow.

Figure 4 illustrates the flow rates at different pumping pressures, where the graph’s slope is the inverse of the fluidic resistance (Rf-1) of that particular fluid line (inner, middle or outer). These data points are a compilation of all data points recorded during the production of double emulsions.

Figure 4. Flow Characteristics of the Double Emulsion System

Droplet Production and Mechanism of Droplet Breakup

Backward flow of the input streams all the way to the fluid reservoir must be avoided to prevent contamination. The 2-way In-line Valves help ensuring that flow rates are always positive.

After setting up the flows, the droplet production at the junction can be observed using the imaging system. A characteristic image of double emulsion production at the junction with bright field microscopy is illustrated in Figure 5.

A one second duration video at high frame rate of the droplet production is documented and split into frames. Figure 6 presents the images with respective time stamps to represent the timeline of droplet breakup.

Droplets flow from left to right. A timescale of 35ms is relative to a droplet production frequency of about 28 per second. The fluid properties and surfactant chemistry have an influence on the production of double emulsions.

Figure 6. Sequence of images depicting the pinch-off during production of double emulsions

Producing Double Emulsions with Variable Inner/Outer Droplet Size

Here, three illustrative cases are presented with constant outer droplet size and varying inner droplet size. For this purpose, it is essential to change all three pressures because the fluids involved are linked by means of fluidic resistance.

For each of these cases, the size of the inner droplet can be tuned by further varying the flows to achieve a tuneable volume percentage of the inner versus outer droplet. Droplets become spherical when collected off-chip, thereby enabling accurate measurement. Results of test 1, 2, and 3 are presented in Figures 7, 8, and 9.

Figure 7. Test 1: Outer Droplet Size 100µm with droplet production at 200 per second

Figure 8. Test 2: Outer Droplet Size 125µm with droplet production at 160 per second

Figure 9. Test 3: Outer Droplet Size 150µm with droplet production at 70 per second

Close packed monolayers of sample on a collection glass slide is illustrated in Figure 10.

Figure 10. Close packed monolayers of sample on a collection glass slide. These images are representative of Test 1, case a, b and c respectively.

Monodispersity of Collected Sample

A droplet size-analysis is conducted by recording droplet diameters for 15 minutes at low frame rate. Dolomite’s Droplet Monitor Software is used to process the recorded images to extract size information, providing temporal data on shifts in droplet sizes.

The data is presented in the distribution based histogram in Figure 11, revealing monodispersity and consistency of the sample for both the outer oil and inner water droplets.

The flow of the three input fluids streams can be set independently using three P-Pumps, facilitating superior control over inner droplet size, outer droplet size, and production frequency.

About Dolomite Microfluidics

Dolomite is a world leader in Productizing Science™ and an innovator in creating microfluidic devices and solutions. We sell the coolest microfluidic products around the world, often working with partner companies to extend the range of technology available to our customers. Productizing Science™ means creating marketable and commercially successful products from scientific discovery, and Dolomite excels in commercialising microfluidic products which exceed expectations.

We offer modular, standard microfluidic systems benefiting a wide range of applications, always adhering to the principles of having multiple functionalities, scalability, user-friendly design and a cost-effective, flexible solution for our customers.

Moreover, we offer Productizing Science™ as a service, which is a product development & manufacturing partnership creating microfluidic solutions for problems which span an extremely wide range of applications. Customers come to Dolomite with their technical challenges, and Dolomite helps solve these problems using its extensive background technology.

Dolomite also designs & manufactures a wide range of world leading standard components such as OEM products, microfluidic connectors & interfaces, chips, pumps, valves, detectors, sensors & accessories. Finally, we offer design consultancy to create customized chips or connectors and/or a prototyping service for the supply of glass, metal or polymer devices, and custom microfluidic connectors.

This information has been sourced, reviewed and adapted from materials provided by Dolomite Microfluidics.